My Motorola DROID 4 has a problem

Out of the blue the button atop the Droid 4 that unlocks or powers down the phone has ceased working.
The phone has not been dropped or misused, well cared for but the button has ceased working.
The button will no longer power down the phone or unlock it.
Luckily I can slide the phone open thus lighting it up and I can unlock and use it.